1. Field of the Invention
The invention relates to the synthesis of compounds (by which it is intended to include monomers, oligomers and polymers) containing the structure Si--N in the molecule. The invention concerns primarily silazanes which are useful to produce ceramic products on pyrolysis but it also relates to compounds which are siloxazanes and/or other compounds containing the Si--group.
2. Description of the Prior Art
Polysilazanes are useful among other things for the preparation of silicon nitride, Si.sub.3 N.sub.4, by pyrolysis. Silicon nitride is a hard material and is useful in forming fibers for reinforcement of composite materials. See, for example, (a) Department of Defense Proceedings. Fourth Metal Matrix Composites Technical Conference, May 19-21, 1981, prepared for DOD Metal Matrix Composites Information Analysis Center and (b) J. J. Brennan, "Program to Study SiC Fiber-Reinforced Glass Matrix Composites, Annual Report to Dept. of Navy (November 1980), Contract No. N00014-78-C-0503.
A number of researchers have developed methods of forming polysilazanes, among them Redl and Rochow, who, in Angew. Chemie. (1964) 76, 650 discuss the preparation of polusilazanes by reaction (1) ##STR1##
Brewer and Haber, J. Am. Chem. Soc. (1948) 70, 3888 and Osthoff and Kantor, Inorg. Syn. (1957) 5, 61 teach the reaction (2) EQU (CH.sub.3).sub.2 SiCl.sub.2 +NH.sub.3 .fwdarw.[(CH.sub.3).sub.2 SiNH].sub.n +HCl (2)
More recent work is described by Markle and others in R. A. Markle, I. Sekercioglu, D. L. Hill, R. R. Wills, and R. G. Sinclair, "Preparation of Si.sub.x N.sub.y C.sub.z Fibers by the Controlled Pyrolysis of Novel Organosilicon Polymeric Precursors", Final Report to NASA, Marshall Flight Center, Alabama, (1981), Contract No. NAS8-33223.
Zoeckler and Laine in J. Org. Chem. (1983) 48, 2539-2541 describe the catalytic activation of the Si--N bond and in particular the ring opening of octamethyl tetrasilazane, ##STR2## and polymerization of the ring-opened intermediate. Chain termination is effected by introducing [(CH.sub.3).sub.3 Si].sub.2 NH as a co-reactant giving rise to polymers (CH.sub.3).sub.3 Si-[NHSi(CH.sub.3).sub.2 ].sub.n -NHSi(CH.sub.3).sub.3 where n may be 1 to 12 or more depending upon the ratio of the chain terminator to the cyclic silazane. The catalyst used was Ru.sub.3 (CO).sub.12. Other publications are as follows: W. Fink, Helv. Chem. Acta., 49, 1408 (1966); Belgian Pat. No. 665774 (1965); Netherlands Pat. No. 6,507,996 (1965); D. Y. Zhinkis et. al., Rus. Chem. Rev., 49, 2814 (1980) and references 51-58; K. A. Andrianov et. al., Dok Akad. Nauk. SSSR, 227, 352 (1976); Dok Akad. Nauk. SSSR, 223, 347 (1975); L. H. Sommer et. al., JACS 91, 7061 (1969); L. H. Sommer, J. Org. Chem. (1967) 32 2470; L. H. Sommer et. al., JACS 89, 5797 (1967).
The methods described in the literature cited above and elsewhere have resulted in one or more of the following disadvantages: low yields of polysilazanes coincident with a high yield of cyclomers, lack of control over product selectivity or quality, etc. Often the product is volatile and is therefore difficult to pyrolyze if ceramic materials are desired from the solid or liquid polymer, or if it is solid, it is an intractable material which cannot be readily shaped, if indeed it can be shaped at all. The product is likely to be contaminated with halogen, especially chloride and it may be extensively cross linked and insoluble. In addition, the high ratio of Si to N in the polymers leads to formation of silicon along with Si.sub.3 N.sub.4 on pyrolysis. In some instances excess carbon and SiC are also produced although they are not always desirable.